Although a variety of membrane key switches for use in keyboards or other data input devices are known, a number of problems relating to the manufacture and use of these switches is common. For instance, incorporating aftertravel into prior art membrane key switch designs has been both difficult and costly.
A sectional view of a typical prior art membrane key switch is shown in FIG. 1. Upper membrane 1 and lower membrane 2 with conductive traces formed thereon are separated by a solid dielectric spacer 3 having an aperture 4. The switch is actuated by depressing key head 5, which forces upper membrane 1 to flex downward through the aperture 4 of spacer 3 and contact stationary lower member 2. Thus, conductive traces on the upper and lower membrane contact each other and complete an electrical circuit. When pressure on the key head 5 is released, return spring 6 returns the key switch to its raised position. This membrane key switch has a serious drawback in that the keyboard base 7 supports the lower membrane 2 and so the key stem 8 cannot move further downward once contact between the conductive traces on the upper and lower membranes is made unless inner spring 9 is present, as described below. The switch makepoint is at the end of switch travel and, consequently, no aftertravel following completion of an electrical circuit is possible.
By having makepoints at the end of switch travel, typical membrane key switches require complete depression of the key in order to make electrical contact. This requirement can cause data entry errors by operators, especially high speed typists who often do not completely depress the keys of a keyboard. In addition, the lack of aftertravel shocks the operator's finger by preventing follow through movement.
An inner membrane depression spring 9 attached to key stem 8 has been used for forcing upper membrane 1 downwards and allowing aftertravel of the key head 5 after the spring 9 forces upper membrane 1 to contact lower membrane 2. However, this design necessitates the use of two springs which adds expense to the manufacture of the switch and aggravates the disadvantages associated with springs, such as loss of rigidity and sensitivity to corrosion.